Soldering is a widely used assembly procedure employed in the manufacture of a variety of products. Soldering, for example, is used in the manufacture of common "tin" cans to seal the side joints, in the assembly of fine jewelry, in the manufacture of automobile bodies and the like. One of the most important uses of soldering and also one of the most critical uses with regard to the quality of the soldering required is in the assembly of electronic apparatus. Soldering is used in the assembly of the electronic apparatus to both physically hold the individual components in place as, for example, on a printed circuit board, and to provide electrical continuity in the circuitry of the apparatus. The solder connections in an electrical assembly must be properly made in order for the resulting electronic apparatus to operate as designed. The problems encountered in soldering electronic apparatus are especially difficult because even relatively simple circuits can literally have hundreds of connections, each of which must be properly soldered. It is furthermore common practice to conduct the soldering of most large circuits in a mass soldering operation wherein all the connections are soldered at the same time by dipping circuitry to be soldered in a molten pool of solder or by passing the circuitry through a wave of molten solder. Mass soldering techniques have proven to be highly successful in increasing the production rate and even in improving the quality of the soldered joints. However, the operating parameters and, particularly, the quality of the solder employed must be closely controlled in order to obtain a satisfactory soldering result on a consistent basis. The single most difficult parameter to maintain in mass soldering processes is the contamination level of the solder.
Solders are made from mixtures of different metals. The particular metal and the relative amounts of each metal in a solder composition are generally selected so as to form a eutectic mixture; that is a mixture having a melting point substantially lower than the melting point of the individual metals used to form the solder. Mixtures of selected metal, for example, lead, tin, bismuth, indium, zinc, silver and/or gold are widely used in solder compositions. The most used solder compositions are mixtures of tin and lead with solders containing about 60% tin and about 40% lead being the principal type of solders used in the soldering of electronic assemblies.
Each solder composition has a certain specific metal composition at which optimum soldering properties are obtained. The presence of even minor amounts of certain metals as impurities can, and often does, have a substantial adverse effect on the solderability properties of the composition. It has been found, however, that it is very difficult to prevent the contamination of the solders, especially in mass soldering processes, such as dip-soldering or wave-soldering, in that the pool of solder is gradually contaminated as a result of material introduced with the circuitry to be soldered, atmospheric impurities and the like. One of the principal sources of metal impurities in solder compositions has been found to be copper, which is inherently removed from circuit boards during the soldering process.
It is recognized that it is important to monitor the solder quality and to maintain solder above certain minimum contamination levels in order to maintain consistent good soldering quality. However, the test methods heretofore proposed for monitoring solder quality were generally unsatisfactory, especially under production conditions. It was suggested, for example, among other things, that the solder be spectrographically analyzed for metal contamination and, particularly, for the presence of certain metallic impurities such as copper. This, however, required relatively expensive test apparatus and required a skilled technician to conduct the evaluation. Furthermore, the test was relatively time-consuming so that often the soldering process was out of control before the test results were obtained. Other suggestions were to measure certain physical properties such as surface tension and the like. The physical measurements, at best, have proven to be highly unreliable. The inability to make a fast, simple and accurate evaluation of the solderability properties has resulted in substantial production losses due to poor soldering.
Solderability problems are also encountered when virgin soldering compositions are used, if changes are made in the process parameters employed in the soldering apparatus, such as changing the temperature of the solder, the soldering rates, fluxes and the like. These changes have a substantial effect on the quality of the resulting soldered connections.
What would be highly desirable would be a relatively simple, fast and accurate process for evaluating solder compositions as to their soldering properties and the effect of process changes on soldering properties.